A commonly used screening body for the top screen of a treatment vessel consists of a plurality of screening bars held together to form a cylinder. Each screening bar has a relatively slight cross-sectional area, about 40 mm.sup.2 (the greatest width being about 4.8 mm and the smallest about 2 mm), and a low section modulus W.sub.y namely below 20 mm.sup.3. A plurality of wires run through transverse holes in the waist portion of the screening bars and the screening bars are surrounded by flat support rings arranged at the same levels as the wires in order to anchor the screening bars to the support rings via said wires. A special connecting element is firmly joined to the radially inner part of each support ring. The connecting element consists of a metal sheet bent to U-shape, the legs of which are welded to the support ring and a cavity is formed for passage of the wire. The connecting element is further provided with radial recesses running vertically through it to receive the foot section of the screening bars. However, the support rings do not give the screening body sufficient stability against torsional stresses and one or more strips must be clamped onto the outside of the screening body, where they extend helically in order to take up torsional stresses in the screening bars during use. Even if the support rings are placed as close together as 70 mm, and several such strips are clamped helically onto the outside, the construction is still unable to withstand the torsional stresses it is subjected to during operation at the top of a pulp digester, for instance, where together with an inner screw it forms a top screen to separate out liquid from the wood chips in a transfer system. The holes in the screening bars contribute to further reduction of the bending resistance (section modulus W.sub.y) of the screening bars. It is thus usual for the screening body to be deformed by the torsional stresses to which it is subjected during operation, due to the influence of the screw and the chips passing through, and also by scrap becoming caught between the screw blades and the screening bars. The entire screening body may be destroyed and must then be replaced with a new one, but even deformation of one or more screening bars will cause disturbances in operation since the screening effect is deteriorated. The known screening body is also complicated to manufacture, the screening bars having to be provided with holes for the wires, the U-shaped connecting elements having to be welded to the screen support rings, special grooves for the foot sections of the screening bars having to be made in the connecting elements, the wires having to be passed alternately through the cavities in the connecting elements and the holes in the screening bars, and the strips running helically around the outside having to be applied with specific pre-stressing in order to give the desired support function. Because of the complicated construction of the screening body, it is not possible to achieve sufficiently small tolerances for the inner diameter of the screening body, which means that the pumping screw to be positioned within the top screen must be subsequently manufactured on the basis of the dimensions of the screening body to ensure a preferred clearance of about 0.5-1 mm. Also, the screening body and screw must therefore be manufactured as a pair and when the screening body must be replaced due to deformation, the screw must also be replaced with a completely new one fitting the new screening body.